When data signals are communicated between circuit devices, reflection of the data signals occurs if the impedances of devices mismatch. Thus, an information signal exchange system requires a terminal resistor that terminates a signal transmission line, and the terminal resistor need to have an impedance matching the impedance of the signal transmission line. The terminal resistor suppresses the reflection of a received signal to raise the integrity of the transferred signals.
The terminal resistor may be located on the inside or outside of a semiconductor chip, which typically has integrated circuits for processing the data signals. The terminal resistor in a semiconductor chip is usually called an on-chip terminator, on-die terminator, or active terminator.
To be effectively used in a semiconductor chip, the on-chip terminator needs to be appropriately controlled according to an operation mode of the semiconductor chip, taken in consideration of power consumption and signal integrity. In other words, the on-die terminator should be controlled differently depending on whether the operation mode is an input mode, an output mode, or a power down mode. The resistance of the on-die terminator should thus be accordingly adjustable.
To obtain a desirable resistance, a plurality of resistors needs to be formed, and a control circuit is used to select resistors from the plurality of resistors and connecting the selected resistors to obtain a desirable resistance. The resulting resistance of the on-die terminator is determined by the number of the selected resistors, and their resistances.
Conventionally, there are two common types of resistors for forming on-die terminators. The first type of resistors is formed of polysilicon (poly) strips, which may be doped with a p-type or an n-type impurity. Polysilicon resistors have the advantageous feature of less prone to process variations, temperature variations, and voltage variations, which are in combination commonly referred to as PVT variations. Therefore, polysilicon resistors have smaller resistance variations from the designed values, typically around about 15 percent or less. However, the maximum currents per unit width that can be endured by polysilicon resistors are low, and hence polysilicon resistors typically occupy big chip areas.
Since polysilicon resistors are typically formed simultaneously with other devices in the same semiconductor chip, with the down-scaling of integrated circuits, polysilicon strips are becoming increasingly thinner, and hence the respective maximum endurable current decreases. However, the off-chip impedance stays the same, requiring the resistances of the on-die terminators to stay the same. As a result, polysilicon resistors need to occupy increasingly greater chip areas.
The other type of on-die terminators are formed of well regions, which may be doped with a p-type or an n-type impurity, typically an n-type impurity, and hence are commonly referred to as n-well resistors. N-well resistors may stand higher current per unit width, and hence can be made smaller. However, n-well resistors are prone to the PVT variations, and may have high resistance variations, for example, around about 50 percent. It is thus difficult to form precise on-die terminators using n-wells.
Existing on-die terminators cannot satisfy both the requirements of reduced chip area usage and high accuracy at the same time. Accordingly, new on-die terminators and methods for forming the same are needed.